This invention relates to the field of wavelength division multiplexing and dense wavelength division multiplexing/demultiplexing, and particularly to the use of reconfigurable diffraction gratings in both fields.
Wavelength Division Multiplexing (WDM) and Dense Wavelength Division Multiplexing (DWDM) systems are important components in fiber optic communication systems and networks. The essential component of such systems focus on the multiplexer/demultiplexer. Current alternatives include diffractive elements such as dielectric thin film filters, arrayed waveguide gratings and fiber Bragg gratings, each possessing their own advantages and disadvantages for a particular system design. Overall it is desirable in such a system or network application to minimize crosstalk between channels by maximizing channel isolation. This task becomes increasingly more difficult in all fiber DWDM systems where channel spacing and bandwidth is very small. (Pan, Shi, and Loh, WDM Solutions, Laser Focus World Supplement, Sep. 1999, pp. 15-18.) Prior art technology is primarily comprised of devices and systems utilizing the aforementioned diffractive elements, and attention if often given to improving resolution of the system while minimizing crosstalk by improving other system components such as the fiber optic channels.
Reconfigurable multiplexing devices can be achieved in a variety of ways. Several such prior art devices have different levels of reconfigurability, functionality and performance. For example, U.S. Pat. No. 5,245,681 discloses a reconfigurable wavelength multiplexing device that relies on a fiber optic tree structured switching matrix to provide the reconfigurability of the system. The switching matrix is comprised of several stages of optical couplers that are controlled by the application of a specified voltage. The voltage application is used to control the passage of light through the coupler. The ability to turn on and off the voltages to the individual couplers in the matrix make the device reconfigurable.
U.S. Pat. No. 5,550,818 utilizes an optical fiber network ring in a wavelength division multiplexing system. Cross-connecting switching devices control the signal routing through the system. Again, the switches are based on voltage application to the switch.
U.S. Pat. No. 5,650,835 discloses a reconfigurable optical beamsplitter in which periods of optical phase shift regions are established across a liquid crystal cell to form an optical grating in the liquid crystal that will perform as a multiplexing or demultiplexing device. The desired pattern of phase shift regions across the cell is accomplished by applying corresponding voltage differentials across the cell which can be dynamically reconfigured.
U.S. Pat. No. 5,771,112 provides a reconfigurable device for the insertion and the extraction of wavelengths utilizing a main optical switch connected to a specified number of add/drop multiplexers.
U.S. Pat. No. 5,712,932 provides a dynamically reconfigurable wavelength division multiplexer. The reconfigurable optical routing system is achieved by using fiber-based Bragg grating or a wavelength selecting optical switch in combination with a fiber optic directional coupler.
All of the above-noted prior art relies primarily on some form of an optical switch to allow or disallow signal passage through an established route in the network or system. However, these systems are limited in their ability to redirect a specified wavelength through a different route in the system. An improvement that enables such rerouting would have direct application in and benefits to the communications industry.
A key component in a WDM/DWDM system is the means of separating the incident light by wavelength. Although there are many means to accomplish this, recent advances in micromachining technology have led to the development of reconfigurable diffraction gratings that can be applied to multiplexing. Such micromachined gratings can be used for various electro-optical applications such as multiplexing, spectroscopy, modulated display technology and optical signal processing.
A deformable grating apparatus is presented in U.S. Pat. Nos. 5,459,610 and 5,311,360, both by Bloom et al. An array of beams, at initially equal heights and with reflective surfaces, are supported at predetermined fractions of incident wavelength above a similarly reflective base. Below the base is a means of electrostatically controlling the position of the beams by supplying an attractive force which will deflect all of the beams or every other beam to a secondary position. The diffraction of the incident light is dependent upon the position of the reflective beam elements.
An electronically programmable diffraction grating is presented in U.S. Pat. No. 5,757,536 by Ricco, et al. A plurality of electrodes control a series of grating elements whose upper surface diffract incident light. The grating is typically formed by a micromachining process.
Although reconfigurable in nature, the micromachined diffraction gratings discussed above are still limited in their useable bandwidth and the span of available wavelengths for at least the specified application of lightwave multiplexing and demultiplexing. Ideally, the reconfigurable diffraction grating used should at least permit virtually unlimited, dynamic wavelength selection, which the prior art does not permit.
Therefore, it is the object of the invention disclosed herein to provide an improved wavelength division multiplexer (WDM) using an electronically reconfigurable diffraction grating.
It is also an object of the invention to provide an improved dense wavelength division multiplexer (DWDM) using an electronically reconfigurable diffraction grating.
It is also an object of the invention to provide an improved WDM/DWDM system using an electronically reconfigurable diffraction grating capable of dynamic output reconfigurability.
It is also an object of the invention to provide an improved WDM/DWDM system capable of simultaneous demultiplexing and optical switching functions.
It is also an object of the invention to provide an improved WDM/DWDM system capable of demultiplexing input light over an increased optical waveband range, and particular, to better enable short wavelength and narrow waveband demultiplexing.
The invention disclosed herein in several embodiments provides an improvement in a wavelength division multiplexer and/or a dense wavelength division multiplexer (WDM/DWDM) by incorporating an electronically reconfigurable compound diffraction grating into the overall multiplexing apparatus or system. The introduction of an electronically reconfigurable compound diffraction grating, which is typically fabricated using MEMS (microelectromechanical systems) technology, improves the compact design, durability, and functionality of the WDM/DWDM system.
In particular, the electronically reconfigurable compound diffraction grating improves upon alternative wavelength separation technology such as dielectric filters, arrayed waveguide gratings, and fiber Bragg gratings, which all limit WDM/DWDM systems regarding channel separation and channel. The present invention allows individual channels or wavelengths to be automatically switched between different system detectors. The optical switching functions, and also the multiplexing/demultiplexing functions, are both incorporated in a single device. This adds tremendous flexibility to an optical network. The reconfigurable compound diffraction grating has the necessary resolution to be useful for both WDM and DWDM applications.
In the preferred embodiment, this optical wavelength division multiplexing apparatus comprises a reconfigurable diffraction grating diffracting at least one input light beam into diffracted light beams of N wavebands wherein N is an integer greater than zero; and further diffracting each of these input light beams into diffracted light beams across X diffraction orders wherein X is an integer greater than zero, for each of these N wavebands.